1. Field of the Invention
This invention relates generally to the in-situ retorting of oil shale. Fluid is injected into the rock surrounding an active in-situ oil shale retort in a manner so that the injected fluid flows toward the active retort to contain high temperature gases and liquids produced during the retorting process, thus reducing heat losses and the flow of contaminants from the retort and maintaining pressure in the retort.
2. Description of the Prior Art
Oil shale may be defined as any fine-grained, compact, sedimentary rock containing organic matter made up mostly of kerogen, a high-molecular-weight solid or semi-solid substance that is insoluble in petroleum solvents and is essentially immobile in its rock matrix. Oil shale rivals coal as the world's most abundant form of hydrocarbon deposit. The presence of large oil shale deposits in the United States has stimulated much effort toward developing methods for recovering liquid and gaseous hydrocarbon products from oil shale.
Several such methods have been proposed which involve the direct application of heat to a subterranean oil shale formation. These methods are collectively known as in-situ retorting of oil shale. In-situ retorting of oil shale has been described in several patents, including U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; 4,091,869; and 4,263,970. U.S. Pat. No. 3,661,423, issued May 9, 1972 to Garret, discloses an in-situ retorting method which involves the steps of mining a void in a subterranean oil shale formation and fragmenting part of the formation near the void to form a defined volume known as a "retort" which contains a stationary, permeable mass of fragmented oil shale particles. The retort is surrounded by an unfragmented rock formation. Garret suggests that hot retorting gases be passed downward through the mass of fragmented oil shale particles to convert the kerogen contained therein into liquid and gaseous hydrocarbon products and other liquids and gases.
The process of Garret results in three zones in the subterranean retort: an upper combustion zone, a retort zone below the combustion zone, and a lower, cooler zone. Garret discloses the production of the hot retorting gases noted above by igniting the upper level of the retort using an initial supply of fuel and air to establish a combustion zone. In the combustion zone the kerogen-containing fragmented oil shale is retorted to produce liquid and gaseous hydrocarbons and oxygen is consumed by burning some of these produced hydrocarbons as well as by burning residual carbon in the retorted oil shale. Hot exhaust gases are produced as the result of the combustion and are used to retort the fragmented oil shale in a retort zone adjacent to and below the combustion zone. After the exhaust gases reach a sufficient temperature, the initial fuel supply is stopped and an oxygen source, such as air, introduced to allow the combustion zone to advance downward through the retort, driving ahead of itself the hot exhaust gases. In the retort zone, the hot exhaust gases decompose the kerogen into liquid and gaseous hydrocarbon products which flow downward and may be collected at the bottom of the retort.
U.S. Pat. No. 4,043,595, issued Aug. 23, 1977 to French, U.S. Pat. No. 4,043,596, issued Aug. 23, 1977 to Ridley, U.S. Pat. No. 4,043,597 issued Aug. 23, 1977 to French, and U.S. Pat. No. 4,043,598, issued Aug. 23, 1977 to French et al., disclose a variety of methods of forming an in-situ oil shale retort in which active retorting may be conducted in the manner disclosed in Garret.
During the active retorting step of in-situ retorting of oil shale, hot gases and liquids may escape to the surrounding rock formation through the rock matrix and fractures therein. Such leakage is aggravated when a series of in-situ retorts are created and hot gases and liquids escape from an active retort site to adjacent abandoned retorts as well as to the surrounding rock not yet retorted. Considerable amounts of heat and reaction products may escape from an active retort as the result of such leakage. Also, leakage of reaction products from an active in-situ retort may increase the difficulty and expense of sustaining retorting operations at a desired pressure. To reduce the amount of energy required to maintain a desired temperature in an active retort, and to reduce the difficulty and expense of maintaining a desired pressure in an active retort, it is important to minimize such loses.
Such hot escaping fluids typically include hazardous chemicals. Considerable contamination of the formation surrounding an active retort may result from such leakage of hazardous chemicals. Additionally, leakage of such hazardous reaction products, from an active retort through mine shafts or fractures may jeopardize the safety of mineworkers engaged in adjacent mining operations.
Current methods for reducing such heat loss, environmental contamination, and mineworker safety problems due to leakage include operating active retorts at low pressure, typically at just below atmospheric pressure, and increasing the spacing of a series of oil shale retorts conducted in a formation. Operation of an oil shale retort at low pressure may have the disadvantage of increasing the required diameter, and hence the expense of conduits needed to withdraw produced fluids out of a retort during active retorting. Increasing the spacing between retorts results in poor utilization of the oil shale resource.
U.S. Pat. No. 4,091,869, issued May 30, 1978 to Hoyer discloses a method of in-situ oil shale retorting in which a series of retorts are sequentially formed. After a first retort is formed, each succeeding retort is formed immediately laterally adjacent to an abandoned, or spent, retort in which active retorting has been completed. Hoyer recognizes that during active retorting, produced gases may leak to a permeable spent retort bordering the active retort. Hoyer proposes compacting the rubble in the spent retort and, either before or after so compacting the rubble, introducing sealing fluids into the rubble in the spent retort to reduce its permeability to the flow of gas. Hoyer suggests the use of aqueous solutions containing such additives as resins, silicates, or hydrated oxides as sealing fluids. Hoyer does not disclose any method for reducing fluid leakage at the interface between an active retort and a surrounding unfragmented rock formation. Hoyer does not acknowledge that a leakage problem may exist at such an interface. Rather, Hoyer characterizes as "impermeable" such surrounding unfragmented rock, though this characterization is not an accurate one for most formations.
It is an object of the present invention to minimize leakage of hot produced gases and liquids from an active in-situ oil shale retort to minimize heat loss and hence to reduce the amount of energy required to maintain the retort at a desired temperature. It is a further object of the present invention to maintain the pressure inside an active in-situ oil shale retort at a desired level to minimize the cost of equipment needed to sustain the retorting process and collect the produced fluids. It is also an object of the present invention to decrease the leakage of hazardous gases from an active in-situ oil retort to reduce contamination of the surrounding formation and to reduce the danger to mineworkers engaged in adjacent mining operations.
An additional benefit of the disclosed invention is that, by reducing hot fluid leakage from an active retort to one or more adjacent abandoned retorts, it allows a series of distinctly formed retorts to be more closely spaced than previously practical to increase the total recovery of shale oil from a given formation.